Semiconductor memory device and method for forming the same

ABSTRACT

A method for forming a semiconductor memory device is provided. The method includes forming a sacrificial via in a dielectric layer over a substrate, forming a first active layer over the dielectric layer, forming an insulating layer over the first active layer, and forming a second active layer over the insulating layer. The method also includes forming a trench through the second active layer, the insulating layer and the first active layer and corresponding to the sacrificial via, removing the sacrificial via to form a via hole in the dielectric layer, and filling the trench and the via hole with a conductive material.

BACKGROUND

The semiconductor integrated circuit (IC) industry has experiencedexponential growth. Technological advances in IC materials and designhave produced generations of ICs where each generation has smaller andmore complex circuits than the previous generation. In the course of ICevolution, functional density (i.e., the number of interconnecteddevices per chip area) has generally increased while geometry size(i.e., the smallest component (or line) that can be created using afabrication process) has decreased. This scaling down process generallyprovides benefits by increasing production efficiency and loweringassociated costs. Such scaling down has also increased the complexity ofprocessing and manufacturing ICs and, for these advancements to berealized, similar developments in IC processing and manufacturing areneeded.

One type of device targeted for increased capacity and integration arememory devices. A reduction in memory device cell design has led tochallenges in interconnect structure providing access and operation tothese memory device cells. Furthermore, the peripheral devices used toaccess these memory device cells have been targeted for improvements inintegration.

Therefore, although conventional semiconductor devices have beengenerally adequate for their intended purposes, they are notsatisfactory in every respect.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying Figures. It shouldbe noted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1 is a schematic diagram of a semiconductor memory device includingan array of memory cells, in accordance with some embodiments of thedisclosure.

FIG. 2A is a cross-sectional view of a semiconductor memory deviceillustrating an interconnection from a peripheral circuit to an array ofmemory cells, in accordance with some embodiments of the disclosure.

FIG. 2B is a layout of access lines of the semiconductor memory deviceof FIG. 2A, in accordance with some embodiments of the disclosure.

FIGS. 3A-1, 3B-1, 3C, 3D-1, 3E-1, 3F-1, 3G-1, 3H-1, 3I-1, 3J-1, 3K,3L-1, 3M-1, 3M-2, 3N-1, 3N-2, 3O-1, 3O-2, 3P-1, 3P-2, 3Q-1, 3Q-2, 3R-1,and 3R-2 are cross-sectional views of the formation of a semiconductormemory device at various intermediate stages, in accordance with someembodiments of the disclosure.

FIGS. 3A-2, 3B-2, 3D-2, 3F-2, 3G-2, 3H-2, 3I-2, 3J-2, 3L-2, 3M-3, 3N-3,3O-3, 3P-3, 3Q-3 and 3R-3 are top views of the formation of asemiconductor memory device at various intermediate stages, inaccordance with some embodiments of the disclosure.

FIGS. 3D-3, 3E-2, 3F-3, 3G-3, 3H-3, 3I-3, and 3J-3 are enlarged views ofarea A shown in FIGS. 3D-1, 3E-1, 3F-1, 3G-1, 3H-1, 3I-1, and 3J-1, inaccordance with some embodiments of the disclosure.

FIG. 4-1 is a cross-sectional view of a semiconductor memory device, inaccordance with some embodiments of the disclosure.

FIG. 4-2 is an enlarged view of area A shown in FIG. 4-1, in accordancewith some embodiments of the disclosure.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the subject matterprovided. Specific examples of components and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a first feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

Some variations of the embodiments are described. Throughout the variousviews and illustrative embodiments, like reference numerals are used todesignate like elements. It should be understood that additionaloperations can be provided before, during, and after the method, andsome of the operations described can be replaced or eliminated for otherembodiments of the method.

Furthermore, when a number or a range of numbers is described with“about,” “approximate,” and the like, the term is intended to encompassnumbers that are within a reasonable range including the numberdescribed, such as within +/−10% of the number described or other valuesas understood by person skilled in the art. For example, the term “about5 nm” encompasses the dimension range from 4.5 nm to 5.5 nm.

Embodiments for forming a semiconductor memory device are provided. Themethod for forming the semiconductor memory device may include forming asacrificial via in a dielectric layer over a substrate, forming a stackof layers for a memory cell array over the dielectric layer, forming atrench through the stack of layers and corresponding to the sacrificialvia, removing the sacrificial via to form a via hole in the dielectriclayer, and filling the trench and the via hole with a conductivematerial. As a result, the overlay window of via to access line may beimproved, high aspect ratio etching may be avoided, and the risk of thedamage of the storage layer of the semiconductor memory device may beprevented. Therefore, the production yield of the semiconductor memorydevice may be increased.

FIG. 1 is a schematic diagram of a semiconductor memory device 100including an array of memory cells, in accordance with some embodimentsof the disclosure. The semiconductor memory device 100 includes asubstrate 102, a peripheral circuit region 110, and a memory cell arrayregion 150, as shown in FIG. 1, in accordance with some embodiments. Theperipheral circuit region 110 is formed vertically above the substrate102, and the memory cell array region 150 is formed vertically above theperipheral circuit region 110, in accordance with some embodiments.

The peripheral circuit region 110 includes one or more peripheralcircuits, e.g., logic circuits, which are illustrated for reference aslogic devices 112A-D, in accordance with some embodiments. Theperipheral circuits are configured for driving the devices of the memorycell array region 150, e.g., through an interconnect structure 120, inaccordance with some embodiments. For example, the peripheral circuitregion 110 may include various devices operable to access and/or controldevices of the memory cell array region (e.g., to performread/write/erase operations).

The devices of the peripheral circuit region 110 may includetransistors, e.g., metal oxide semiconductor field effect transistors(MOSFET) such as p-channel and/or n-channel MOSFETs, complementary metaloxide semiconductor (CMOS) transistors, bipolar junction transistors(BJT), high-voltage transistors, high-frequency transistors, etc. Thedevices of the peripheral circuit region 110 may be configured as planartransistors or multi-gate transistors such as FinFET devices,gate-all-around (GAA) devices, etc.

The memory cell array region 150 includes a plurality of arrays ofmemory cells, which are illustrated for reference as memory cell array152A and 152B, in accordance with some embodiments. Each of the memorycell arrays 152A and 152B includes a plurality of memory cells 154operable for storage, in accordance with some embodiments. The memorycell arrays 152A and/or 152B include stackable memory cells 154,vertically arranged in an array format, in accordance with someembodiments. The memory cells 154 are arranged in a row/columnconfiguration, in accordance with some embodiments. The memory cells 154are flash memory transistors, e.g., with SONOS(silicon-oxide-nitride-oxide-silicon) structure, in accordance with someembodiments. In some embodiments, the memory cells are NOR-type flashmemory cells.

The semiconductor memory device 100 is referred to as a peripheralcircuit under memory array (PUA) device, in accordance with someembodiments. The PUA device configuration may provide an increase inmemory cell density. For example, the increase may be evident incomparison to a configuration positioning a peripheral circuit regionlaterally adjacent (e.g., side-by-side) with the memory cell arrays. Incontrast to the “side-by-side” configuration, the PUA device may allowthe memory cells to be formed at least partially vertically above theperipheral circuitry.

The design and implementation of the interconnect structure 120 betweenthe memory cell array region 150 and the peripheral circuit region 110in a peripheral circuit under array configuration however may bechallenging. For example, the metal layer/via routing in theinterconnect structure 120 between the memory cell array region 150 andthe peripheral circuit region 110 must be addressed. FIGS. 2A and 2Bprovide an illustration of such a routing design, in accordance withsome embodiments.

FIG. 2A is a cross-sectional view of a semiconductor memory device 200illustrating an interconnect structure from a peripheral circuit to anarray of memory cells, in accordance with some embodiments of thedisclosure. FIG. 2B is a layout of access lines of the semiconductormemory device 200, in accordance with some embodiments of thedisclosure.

The semiconductor memory device 200 is a PUA device, in accordance withsome embodiments. The semiconductor memory device 200 includes aperipheral circuit region 110 formed on a substrate 102, a first (orlower) interconnect structure 120 vertically above the peripheralcircuit region 110, and a memory cell array region 150 vertically abovethe first interconnect structure 120, as shown in FIG. 2A, in accordancewith some embodiments.

The first interconnect structure 120 is a multi-layer interconnect (MLI)used to connect logic devices 112A and 112B of the peripheral circuitregion 110 with one another, in accordance with some embodiments. TheMLI of the first interconnect structure 120 is also used to connect thelogic devices 112A and/or 112B of the peripheral circuit region 110 withdevices of the memory cell array region 150 (e.g., memory cells 154 andcomponents thereof), in accordance with some embodiments.

The MLI of the interconnect structure 120 includes a plurality of metallayers 128 (e.g., providing horizontal routing), and a plurality ofcontacts 124 and vias 130 (e.g., providing vertical routing), as shownin FIGS. 2A and 2B, in accordance with some embodiments. The contacts124 are surrounded by an interlayer dielectric (ILD) layer 122 and landon the logic device 112A and 122B, in accordance with some embodiments.The metal layers 128 and the vias 130 are surrounded by an inter-metaldielectric (IMD) layer 126 and connected to the contacts 124, inaccordance with some embodiments.

The uppermost or top metal layer 128 of the MLI includes a plurality ofaccess lines 128M, as shown in FIGS. 2A and 2B, in accordance with someembodiments. The access lines 128M are planar with one another (e.g., atthe same metallization level), in accordance with some embodiments. Theaccess lines 128M are conductive lines operable to access the memorycells 154 of the memory cell array region 150, in accordance with someembodiments. The access lines 128M are access lines providing electricalconnection to the memory cells 154 of the memory cell array region 150,in accordance with some embodiments. In some embodiments, the accesslines 128M provide word lines (WL) for accessing the memory cells 154 ofthe memory cell array region 150. In some embodiments, the access lines128M may provide bit lines (BL) for accessing the memory cells 154 ofthe memory cell array region 150.

The memory cell array region 150 includes array 152A which includes aplurality of memory cells 154, in accordance with some embodiments. Thememory cells 154 are arranged in a row and/or column configuration,e.g., memory cell columns 154A-154G that together operate as a singlememory cell array 152A, in accordance with some embodiments. Each of thememory cell columns 154A-154G are a vertically configured stack ofmultiple memory cells 154 _(1-n), (e.g., 154A₁-154A₄, 154B₁-154B₄,etc.).

The access lines 128M provided by the first interconnect structure 120are interconnected to a first grouping of the memory cells, whichincludes columns 154A, 154C, 154E, 154G, through the vias 130M, inaccordance with some embodiments. That is, the access line 128M isinterconnected to every other memory cell column of the array 152A, inaccordance with some embodiments. Multiple memory cells 154 ₁₋₄ of asingle memory cell columns 154A, 154C, 154E, or 154G share a singlevertically extending gate line (discussed below) that is connected to asingle access line 128M through the via 130M, in accordance with someembodiments. FIG. 2B illustrates an interconnection of a single accessline 128M to the corresponding first grouping of the memory cellcolumns, and one of ordinary skill may recognize interconnections ofother access lines 128M to the corresponding first grouping of thememory cell columns.

A second (or upper) interconnect structure 220 is formed verticallyabove the memory cell array region 150, as shown in FIGS. 2A and 2B, inaccordance with some embodiments. The second interconnect structure 220include vias (e.g., providing vertical routing) and metal layers (e.g.,providing horizontal routing), in accordance with some embodiments. Themetal layers and vias are surrounded by an IMD layer 226, in accordancewith some embodiments. It should be noted that the second interconnectstructure 220 is connected the logic devices 112A and 112B of theperipheral circuit region 110, for example, through the firstinterconnect structure 120 in another region of the substrate 102 (notshown), in accordance with some embodiments.

The lowermost or bottom metal layers of the second interconnectstructure 220 include a plurality of access lines 228N, as shown inFIGS. 2A and 2B, in accordance with some embodiments. The access lines228N are planar with one another (e.g., at the same metallizationlevel), in accordance with some embodiments. The access lines 228N areconductive lines operable to access memory cells 154 of the memory cellarray region 150, in accordance with some embodiments. The access lines228N are access lines providing electrical connection to the memorycells 154 of the memory cell array region 150, in accordance with someembodiments. In some embodiments, the access lines 228N provide wordlines (WL) for accessing memory cells 154 of the memory cell arrayregion 150. In some embodiments, the access lines 228N may provide bitlines (BL) for accessing memory cells 154 of the memory cell arrayregion 150. In some embodiments, the access lines 128M and 228N providethe same functionality (e.g., both provide word lines). In someembodiments, the gate pick-up of the memory cells of the memory cellarray is performed either by interconnection to one of the access lines128M or the access lines 228N.

The access lines 228N provided by the second interconnect structure 220are interconnected to a second grouping of the memory cells, whichincludes columns 154B, 154D, 154F, through the vias 230N, in accordancewith some embodiments. That is, the access line 228N is interconnectedto every other memory cell column of the array 152A, in accordance withsome embodiments. Multiple memory cells 154 ₁₋₄ of a single memory cellcolumns 154B, 154D, or 154F share a single vertically extending gateline (discussed below) that is connected to a single access line 228Nthrough the via 230N, in accordance with some embodiments. FIG. 2Billustrates an interconnection of a single access line 228N to thecorresponding second grouping of the memory cell columns, and one ofordinary skill may recognize interconnections of other access lines 228Nto the corresponding second grouping of the memory cell columns.

In some embodiments, the memory cells 154 have a Y-pitch P_(C-Y) (in theY direction) and an X-pitch P_(C-X) (in the X direction) betweenadjacent memory cells 154, as shown in FIG. 2B. In some embodiments, theY-pitch P_(C-Y) of the memory cells 154 is defined by the WL pitch, andthe X-pitch P_(C-X) of the memory cells 154 is defined by the BL pitch.

The routing illustrated in FIGS. 2A and 2B may be advantageous in thatit can avoid having all interconnection of the access lines (e.g., gatepick-ups) through a top of the memory cell array, which would degradememory cell scalability. Because half of the interconnections with thememory cells (e.g., gate pick-ups) being done at top region of the array(e.g., through access line 228N) and the other half of theinterconnections with the memory cells (e.g., gate pick-ups) being doneat a bottom region of the array (e.g., through access line 128M), thepitch required for interconnection of the access lines of thesemiconductor memory device 200 may be relaxed, which may allow forimproved scalability of the array.

The lower interconnect structure 120 or portions thereof between theperipheral circuit region 110 and the memory cell array region 150 arereferred to as an interposer, in accordance with some embodiments. Theinterposer includes all or portions of, for example, the MLI ofinterconnect structure 120 such as vias 130M and/or the top metal layer128 including access lines 128M, in accordance with some embodiments.Thus, various aspects of the present disclosure provide an interposerformation method before the memory cell array process to providebenefits, in some embodiments, one or more of (1) simplifying processintegration flow, (2) relaxing interconnect (e.g., metal) pitchrequirements, and/or (3) improving memory performance, (e.g., bandwidth(BW) of 3D flash memory).

FIGS. 3A-1, 3B-1, 3C, 3D-1, 3E-1, 3F-1, 3G-1, 3H-1, 3I-1, 3J-1, 3K,3L-1, 3M-1, 3M-2, 3N-1, 3N-2, 3O-1, 3O-2, 3P-1, 3P-2, 3Q-1, 3Q-2, 3R-1,and 3R-2 are cross-sectional views of the formation of a semiconductormemory device 300 at various intermediate stages, in accordance withsome embodiments of the disclosure. FIGS. 3A-2, 3B-2, 3D-2, 3F-2, 3G-2,3H-2, 3I-2, 3J-2, 3L-2, 3M-3, 3N-3, 3O-3, 3P-3, 3Q-3 and 3R-3 are topviews of the formation of the semiconductor memory device 300 at variousintermediate stages, in accordance with some embodiments of thedisclosure. FIGS. 3D-3, 3E-2, 3F-3, 3G-3, 3H-3, 3I-3, and 3J-3 areenlarged views of area A shown in FIGS. 3D-1, 3E-1, 3F-1, 3G-1, 3H-1,3I-1, and 3J-1, in accordance with some embodiments of the disclosure.

Furthermore, FIGS. 3A-1, 3B-1, 3C, 3D-1, 3E-1, 3F-1, 3G-1, 3H-1, 3I-1,3J-1, 3K, 3L-1, 3M-1, 3N-1, 3O-1, 3P-1, 3Q-1, and 3R-1 arecross-sectional views taken along line I-I of FIGS. 3A-2, 3B-2, 3D-2,3F-2, 3G-2, 3H-2, 3I-2, 3J-2, 3L-2, 3M-3, 3N-3, 3O-3, 3P-3, 3Q-3 and3R-3, in accordance with some embodiments of the disclosure. FIGS. 3M-2,3N-2, 3O-2, 3P-2, 3Q-2, and 3R-2 are cross-sectional views taken alongline II-II of FIGS. 3M-3, 3N-3, 3O-3, 3P-3, 3Q-3 and 3R-3, in accordancewith some embodiments of the disclosure.

A substrate 302 is provided, as shown in FIG. 3A-1, in accordance withsome embodiments. In some embodiments, the substrate 302 is asemiconductor substrate such as a silicon substrate. In someembodiments, the substrate 302 includes an elementary semiconductor suchas germanium; a compound semiconductor such as gallium nitride (GaN),silicon carbide (SiC), gallium arsenide (GaAs), gallium phosphide (GaP),indium phosphide (InP), indium arsenide (InAs), and/or indium antimonide(InSb); an alloy semiconductor such as SiGe, GaAsP, AlInAs, AlGaAs,GalnAs, GaInP, and/or GaInAsP; or a combination thereof. Furthermore,the substrate 302 may optionally include an epitaxial layer (epi-layer),may be strained for performance enhancement, may include asilicon-on-insulator (SOI) structure, and/or have other suitableenhancement features.

An isolation structure 304 is formed in the substrate 302 to defineactive regions of the substrate 302, as shown in FIG. 3A-1, inaccordance with some embodiments. In some embodiments, the isolationstructure 304 is made of one or more dielectric materials, such assilicon oxide, silicon nitride, silicon oxynitride (SiON), anothersuitable dielectric material, and/or a combination thereof. In someembodiments, the steps of forming the isolation structure 304 includerecessing the substrate 302 to form trenches, depositing one or moredielectric materials in the trenches and over the upper surface of thesubstrate 302, and planarizing the one or more dielectric materials overthe upper surface of the substrate 302 using, for example, chemicalmechanical polishing (CMP). The deposition may be chemical vapordeposition (CVD) (such as low pressure CVD (LPCVD), plasma enhanced CVD(PECVD), high density plasma CVD (HDP-CVD), high aspect ratio process(HARP), and flowable CVD (FCVD)), atomic layer deposition (ALD), spin-oncoating, another suitable method, or a combination thereof.

Logic devices 312A and 312B are formed on the substrate 302 in theactive regions, as shown in FIG. 3A-1, in accordance with someembodiments. The logic devices 312A and 312B are components of theperipheral circuit region (discussed above respect with to FIGS. 1 and2A), in accordance with some embodiments. The peripheral circuitcomponents may make up a control circuit for operating an array ofmemory cells formed vertically above. The peripheral circuit componentsmay include, but is not limited to, voltage boost circuitry, page buffercircuitry, column decoder, row decoder, error correction circuitry,write assist circuitry, interface circuitry including for interfacingbetween types of memory cells, bus control circuitry, and the like.

The logic devices are MOSFETs, in accordance with some embodiments. TheMOS transistors may be p-type MOSFETs (P-MOSFET) or n-type MOSFETs(N-MOSFET). The MOSFET may be planar-type transistors, fin-typetransistors (e.g., FinFETs), and/or other transistor configurationsincluding as discussed above in FIG. 1. In some embodiments, the logicdevices 312A and 312B are planar-type transistors. The logic devices312A and 312B each include a gate structure 316 and source/drain regions314, in accordance with some embodiments. The gate structure 316 isformed over the upper surface of the substrate 302, in accordance withsome embodiments. The source/drain regions 314 are formed in or embeddedat least partially in the substrate 302 on opposite sides of the gatestructure 316, in accordance with some embodiments.

In some embodiments, the gate structure 316 includes a gate dielectriclayer and a gate electrode layer over the gate dielectric layer. In someembodiments, the gate dielectric layer includes an interfacial layer ofdielectric material such as silicon oxide (SiO₂), HfSiO, or siliconoxynitride (SiON). The interfacial layer may be formed using chemicaloxidation, thermal oxidation, ALD, CVD, and/or another suitable method.In some embodiments, the gate dielectric layer includes high-K gatedielectric layer of high-K dielectric materials such as hafnium oxide(HfO₂), TiO₂, HfZrO, Ta₂O₃, HfSiO₄, ZrO₂, ZrSiO₂, LaO, AlO, ZrO, TiO,Ta₂O₅, Y₂O₃, SrTiO₃ (STO), BaTiO₃ (BTO), BaZrO, HfZrO, HfLaO, HfSiO,LaSiO, AlSiO, HfTaO, HfTiO, (Ba,Sr)TiO₃ (BST), Al₂O₃, Si₃N₄, oxynitrides(SiON), combinations thereof, or another suitable material. The high-Kgate dielectric layer may be formed by ALD, physical vapor deposition(PVD), CVD, thermal oxidation, and/or another suitable method.

In some embodiments, the gate electrode layer includes a conductivematerial, such as doped semiconductor, a metal, metal alloy, or metalsilicide. In some embodiments, the gate electrode layer includes asingle layer or alternatively a multi-layer structure, such as variouscombinations of a metal layer with a selected work function to enhancethe device performance (work function metal layer), a liner layer, awetting layer, an adhesion layer, a metal fill layer, and/or anothersuitable layer. The gate electrode layer may be formed of polysilicon,germanium, Ti, Ag, Al, TiAlN, TaC, TaCN, TaSiN, Mn, Zr, TiN, TaN, Ru,Mo, Al, WN, Cu, W, Re, Ir, Co, Ni, another suitable conductive material,or multilayers thereof. The gate electrode layer may be formed by ALD,PVD, CVD, e-beam evaporation, or another suitable process. Furthermore,the gate structures 316 may be formed separately for N-FET and P-FETtransistors which may use different gate electrode layers.

In some embodiments, the source/drain regions 314 are regions of thesubstrate 302 suitably doped using an implantation process. In someembodiments, the source/drain regions 314 are epitaxially grownsource/drain regions using an epitaxial growth process.

A multi-layer interconnect structure 320 is formed over the substrate302, as shown in FIG. 3A-1, in accordance with some embodiments. Theinterconnect structure 320 is formed over and coupled to the logicdevices 312A and 312B, in accordance with some embodiments. Theinterconnect structure 320 includes contacts 322 in an ILD layer 324,and metal layers 328 and conductive vias 330 in multiple IMD layers 326,as shown in FIG. 3A-1, in accordance with some embodiments. Theinterconnect structure 320 serves to interconnect devices (e.g., logicdevices) of the peripheral circuit, in accordance with some embodiments.The interconnect structure 320 serves to interconnect the underlyingperipheral circuit with an overlying memory cell array, in accordancewith some embodiments. As such, portions of the interconnect structure320 may be referred to as providing an interposer.

The ILD layer 322 is formed over the upper surface of the substrate 302and covers the logic devices 312A and 312B, in accordance with someembodiments. In some embodiments, the ILD layer 322 is made of adielectric material, such as tetraethylorthosilicate (TEOS) oxide,un-doped silicate glass (USG), or doped silicon oxide such asborophosphosilicate glass (BPSG), fluoride-doped silicate glass (FSG),phosphosilicate glass (PSG), borosilicate glass (BSG), and/or anothersuitable dielectric material. In some embodiments, the ILD layer 322 isformed using CVD (such as HDP-CVD, PECVD, or HARP), ALD, anothersuitable method, and/or a combination thereof.

The contacts 324 are formed through ILD layer 322 and land on thesource/drain regions 314 of the logic device 312A and 312B, inaccordance with some embodiments. In some embodiments, the contacts 324land on the gate structure 316 (not shown). In some embodiments, thecontacts 324 are made of a one or more conductive materials, forexample, cobalt (Co), nickel (Ni), tungsten (W), titanium (Ti), tantalum(Ta), cupper (Cu), aluminum (Al), ruthenium (Ru), molybdenum (Mo), TiN,TaN, and/or a combination thereof. In some embodiments, the contacts 324include a silicide layer, such as WSi, NiSi, TiSi or CoSi, formed on thesource/drain regions 314. In some embodiments, the formation of thecontacts 324 includes patterning the ILD layer 322 to form contactopenings through the ILD layer 322 and exposing the source/drain regions314. In some embodiments, the conductive material for the contacts fillsthe contact openings and is formed over the upper surface of the ILDlayer 322. In some embodiments, a planarization process such as CMP isperformed on the conductive material until the upper surface of the ILDlayer 322 is exposed.

The multiple IMD layers 326 are formed over the ILD layer 322, inaccordance with some embodiments. In some embodiments, each of the IMDlayers 326 is made of one or more dielectric materials, such as siliconoxide (SiO₂), silicon nitride (SiN), silicon oxynitride (SiON), siliconcarbon nitride (SiCN), oxygen-doped silicon carbide (SiC:O),oxygen-doped silicon carbon nitride (SiCN:O), silicon oxycarbide (SiOC),tetraethylorthosilicate (TEOS) oxide, un-doped silicate glass (USG),borophosphosilicate glass (BPSG), fluoride-doped silicate glass (FSG),phosphosilicate glass (PSG), borosilicate glass (BSG), organosilicateglasses (OSG), Spin-On-Glass, Spin-On-Polymers, silicon carbon material,or a combination thereof. In some embodiments, the IMD layers 326 areformed using CVD (such as LPCVD, PECVD, HDP-CVD, HARP, and FCVD), ALD,spin-on coating, another suitable method, or a combination thereof.

The metal layers 328 and the vias 330 are formed in the IMD layers 326,in accordance with some embodiments. The metal layers 328 provide ahorizontal routing for a signal or signals produced by the peripheralcircuit component (e.g., logic devices 312A and 312B), in accordancewith some embodiments. The vias 330 provide a vertical routing for asignal or signals produced by the peripheral circuit component (e.g.,logic devices 312A and 312B), in accordance with some embodiments.

In some embodiments, the metal layers 328 and the vias 330 are made ofcopper (Cu), cobalt (Co), ruthenium (Ru), molybdenum (Mo), tungsten (W),titanium (Ti), another suitable conductive material, an alloy thereof,nitrides of these materials, multilayers thereof, and/or a combinationthereof. In some embodiments, the metal layers 328 and the vias 330 areformed using damascene processes (e.g., single damascene or dualdamascene), for example, forming openings for the vias 330 and trenchesfor the metal layers 328 in the IMD layers 326, and forming conductivematerial in the openings and the trenches using such as anelectroplating (ECP) process or an electroless depositing (ELD) process.

The uppermost or top metal layer 328 formed provides a plurality ofaccess lines 328M (or referred to bottom access lines 328M), as shown inFIGS. 3A-1 and 3A-2, in accordance with some embodiments. An uppermostor top IMD layer 326M is formed over the access lines 328M, inaccordance with some embodiments. The access lines 328M extend in the Xdirection, in accordance with some embodiments. The access lines 328Mare arranged in the Y direction and spaced from one another, inaccordance with some embodiments. The access lines 328M are conductivelines operable to access and/or control one or more memory cells of thememory cell array, in accordance with some embodiments. In someembodiments, the access lines 328M are word lines (WL) and have a pitchP_(a1-Y) in the Y direction between adjacent word lines. In someembodiments, the access lines 328M are bit lines (BL). It should benoted that whether the access lines 328M are word lines or bit lines isdependent on the desired configuration of the memory cells, where thefunctionality of the other one of the word line or bit line is provided,for example, within the subsequently formed stack of active layers.

Sacrificial vias 332 are formed in the IMD layer 326M, as shown in FIGS.3B-1 and 3B-2, in accordance with some embodiments. The sacrificial vias332 lands on the access lines 328M, in accordance with some embodiments.The sacrificial vias 332 are aligned over each access line 328M, inaccordance with some embodiments. As such, in some embodiments, thesacrificial vias 332 have a Y-pitch P_(V1-Y) (in the Y direction)between adjacent sacrificial vias 332 that is substantially equal topitch P_(a1-Y) of the access line 328M. In some embodiments, thesacrificial vias 332 have an X-pitch P_(V1-X) (in the X direction)between adjacent sacrificial vias 332. In some embodiments, the X-pitchP_(V1-X) of the sacrificial vias 332 is twice the pitch of the bit lines(discussed below). This relaxed X-pitch P_(V1-X) is due to half of thememory cells being configured to connect to the access line 328M underthe memory cell array, and half of the memory cells being configured toconnect to the metal access line above the memory cell array (discussedbelow).

In some embodiments, the sacrificial vias 332 have a dimension Lmeasured in the X direction and a dimension W measured in the Ydirection. The dimension L and the dimension W may be substantiallysimilar. In some embodiments, the dimension L is 0.2 to 0.8 the pitch ofbit lines (discussed below). In some embodiments, the dimension W ofabout 0.2 to about 0.8 the pitch of the word lines (e.g., the pitchP_(a1-Y) of the access lines 328M). In some embodiments, the sacrificialvias 332 have a thickness ranging from about 30 nm to about 200 nm.

The sacrificial vias 332 are made of a dielectric material having adifferent etching selectivity from adjacent dielectric materials (e.g.,uppermost IMD layer 326M), in accordance with some embodiments. In someembodiments, the sacrificial vias 332 are formed of silicon nitride,silicon oxide, and silicon oxynitride. The sacrificial vias 332 may bereferred to as a dielectric interposer.

In some embodiments, the steps of forming the sacrificial vias 332include forming a patterned mask layer (not shown) on the IMD layer326M, and etching the IMD layer 326M uncovered by the patterned masklayer. For example, a photoresist may be formed on the IMD layer 326M,such as by using spin-on coating, and patterned with a patterncorresponding to the sacrificial vias 332 by exposing the photoresist tolight using an appropriate photomask. Exposed or unexposed portions ofthe photo resist may be removed depending on whether a positive ornegative resist is used. The pattern of the photoresist may then betransferred to the IMD layer 326M to form via holes (not shown), such asby using one or more suitable etch processes. The photoresist may beremoved in an ashing or wet strip process, for example. The etchprocesses may include a reactive ion etch (RIE), neutral beam etch(NBE), inductive coupled plasma (ICP) etch, the like, or a combinationthereof. The etch processes may be anisotropic. The etch processes maybe performed until the access lines 328M are exposed.

In some embodiments, afterward, the via holes are filled with adielectric material to form sacrificial vias 332 in the IMD layer 326M.The dielectric material may also be formed over the IMD layer 326M. Insome embodiments, the dielectric material over the upper surface of theIMD layer 326M is removed using a planarization process such as CMP oretching-back until the upper surface of the IMD layer 326M is exposed.

An etching stop layer (ESL) 329 is formed over the upper surface of theuppermost IMD layer 326M, as shown in FIGS. 3B-1 and 3B-2, in accordancewith some embodiments. The etching stop layer 329 covers the sacrificialvias 332, in accordance with some embodiments. The etching stop layer329 interfaces each of the sacrificial vias 332 and the adjacent IMDlayer 326M. Generally, an ESL may provide a mechanism to stop or slowdown an etching process when forming, e.g., openings, holes, trenches,etc. The ESL may be formed of a dielectric material having a differentetching selectivity from adjacent layers or components.

In some embodiments, the etching stop layer 329 is made of a dielectriclayer, such as AlO, AlN, SiOC, or a combination thereof. In someembodiments, the etching stop layer 329 is formed using CVD (such asLPCVD, PECVD, HDP-CVD, HARP, and FCVD), ALD, another suitable method, ora combination thereof.

In some embodiments, the etching stop layer 329 has a thickness rangingfrom about 10 nm to about 200 nm. The thickness of the etching stoplayer 329 may be determined by the number of memory cells verticallystacked above the etching stop layer 329. For example, the more memorycells that are provided vertically above the etching stop layer 329 thegreater the thickness desired for the etching stop layer 329.

A plurality of stacks of layers 334 is formed over the etching stoplayer 329, as shown in FIG. 3C, in accordance with some embodiments. Thestacks 334 are used to form one or more memory cell arrays, inaccordance with some embodiments. Each of the stacks 334 includes, inorder, from bottom to top, an active layer 336, an insulating layer 338,an active layer 340, and an insulating layer 342, in accordance withsome embodiments. The stacks 334 may be repeated any number of timessuch as 2, 4, 6, 8, 16, 24, 32, or more, that is dependent upon thedesired array size. FIG. 3 illustrates 4 cycles of the stacks 334, inaccordance with some embodiments.

The active layers 336 and 340 interposed by the insulating layer 338 arematerial layers used to form source/drain features of vertical-typememory transistors (e.g., channel current flows in the Z direction), inaccordance with some embodiments. The insulating layer 342, theuppermost layer of a single stack 334, is used to isolate the underlyingmemory transistors from the overlying memory transistors, which areformed from the stack 334 of the next cycle, in accordance with someembodiments.

In some embodiments, the active layers 336 and 340 are made ofsemiconductor material, such as silicon (e.g., polysilicon), germanium(e.g., poly-germanium), or another suitable semiconductor material. Theactive layers 336 and 340 are formed using epitaxial growth process,CVD, ALD, or another suitable deposition technique. In some embodiments,the active layers 336 and 340 are doped, e.g., with an n-type or ap-type dopant.

The insulating layer 338 is formed interposing the active layers 336 and340, in accordance with some embodiments. In some embodiments, theinsulating layer 338 is made of an oxide (e.g., silicon oxide). In someembodiments, the insulating layer 338 is formed using CVD (such asPECVD), ALD, another suitable method, or a combination thereof. In someembodiments, the insulating layer 342 is made of a different materialthan the insulating layer 338, for example, a nitride (e.g., siliconnitride). In some embodiments, the insulating layer 342 is formed usingCVD (such as PECVD), ALD, another suitable method, or a combinationthereof.

A capping layer 343 is formed over the upper surface of the uppermoststack 334, as shown in FIG. 3C, in accordance with some embodiments. Thecapping layer 343 is used to protect the stacks 334 in the followingetching processes, in accordance with some embodiments. In someembodiments, the capping layer 343 is made of an oxide (e.g., siliconoxide). In some embodiments, the capping layer 343 is formed using CVD(such as PECVD), ALD, another suitable method, or a combination thereof.

A plurality of trenches 344 is formed through the capping layer 343 andthe plurality of stacks 334, as shown in FIGS. 3D-1, 3D-2, and 3D-3, inaccordance with some embodiments. The trenches 344 penetrate througheach layer of the stacks 334 and expose portions of the upper surface ofthe etching stop layer 329, in accordance with some embodiments. Thetrenches 344 extend in the Y direction, in accordance with someembodiments. The trenches 344 are arranged in the X direction and spacedfrom one other, in accordance with some embodiments. Every other trench344 corresponds to and is aligned with the sacrificial vias 344 arrangedin the Y direction, in accordance with some embodiments. The trenches344 define regions for gate structures (or gate lines) to be formed. Insome embodiments, the trenches 344 are formed using a patterning processincluding a photolithography process followed by an etching process(e.g., dry etching).

Furthermore, the plurality of stacks 334 cut by the trenches 344 forms aplurality of source/drain features 335, 337, 339, 341, as shown in FIG.3D-3, in accordance with some embodiments. The source/drain feature 335and the source/drain feature 339 are formed at a first, or left, side ofa single trench 334 and interposed by the insulating layer 338, inaccordance with some embodiments. The source/drain feature 337 and thesource/drain feature 342 are formed at a second, or right, side of asingle trench 334 and interposed by the insulating layer 338, inaccordance with some embodiments. In some embodiments, the source/drainfeatures 335 and 337 are source features of memory transistors, and thesource/drain features 339 and 341 are drain features of memorytransistors.

In some embodiments, the source/drain features 335 and 337 providesource lines. In some embodiments, the source/drain features 339 and 341provide bit lines (BL) which have a pitch P_(a2-X) in the X directionbetween adjacent bit lines. As discussed above, in some embodiments, thedimension L of the sacrificial via 332 (see FIG. 3B-2) is about 0.2 toabout 0.8 the pitch of bit lines (e.g., the pitch P_(a) _(2-X) ).Furthermore, the X-pitch P_(V1-X) of the sacrificial vias 332 (see FIG.3B-2) is twice the BL pitch P_(a2-X) (e.g., the sacrificial via 332being disposed directly below every other trench 344).

In some embodiments, the source/drain features 339 and 341 provide wordlines (WL). It should be noted that whether the source/drain features339 and 341 are bit lines or word lines is dependent on the desiredconfiguration of the memory cells, where the functionality of the otherone of the word lines or the bit lines are provided, for example, by theaccess lines 328M, routed on the top metal layer 328.

After the trenches 344 are formed, the memory cells C are defined, inaccordance with some embodiments. In some embodiments, the memory cellsC have a Y-pitch P_(C-Y) (in the Y-direction) and an X-pitch P_(C-X) (inthe Z-direction) between horizontally adjacent memory cells C. In someembodiments where the access lines 328M provide word lines and thesource/drain features 339 and 341 provides bit lines, the Y-pitchP_(C-Y) is equal to the WL pitch P_(a1-Y) and an X-pitch P_(C-X) isequal to the BL pitch P_(a2-X). One of ordinary skill may recognize thatthe memory cells C have a Z-pitch between vertically adjacent memorycells C and the Z-pitch is defined by the stacks 334. In someembodiments, a single memory cell C includes two opposite memorytransistors sharing a common gate line (discussed below).

Channel features 346 are formed on the sidewalls of the insulatinglayers 338 exposed from the trenches 344, as shown in FIGS. 3E-1 and3E-2, in accordance with some embodiments. The channel features 346 areformed between the source/drain features 335 and 339 and thesource/drain features 337 and 341, in accordance with some embodiments.The channel features 346 are used as the channel regions of memorytransistors, in accordance with some embodiments.

In some embodiments, the channel features 346 are made of semiconductormaterial, such as silicon (e.g., polysilicon), germanium (e.g.,poly-germanium), or another suitable semiconductor material. Thesemiconductor material may be doped, e.g., with an n-type or a p-typedopant. In some embodiments, the steps of forming the channel features346 include laterally recessing the insulating layers 338 using etchingprocess, conformally depositing a semiconductor material for channelfeatures 346 to fill the recess. The semiconductor material may also beformed on the respective exposed sidewalls of the source/drain features335, 337, 339, 341 and the insulating layer 342. Afterward, thesemiconductor material formed on the exposed sidewalls of thesource/drain features 335, 337, 339, 341 and the insulating layer 342 isetched away thereby leaving portions of the semiconductor material inthe recesses as the channel features 346.

A storage layer 348 is formed over the semiconductor memory device 300,as shown in FIGS. 3F-1, 3F-2, and 3F-3, in accordance with someembodiments. The storage layer 348 functions to trap charges where thecurrent differences detected in the cell (e.g., drain current) providethe memory effect, in accordance with some embodiments. The storagelayer 348 is conformally formed along the sidewalls (e.g, respectivesidewalls of the source/drain features 335, 337, 339, 341, the channelfeature 346, and the insulating layer 342 exposed from the trenches 344)and the bottom surfaces (e.g., the upper surface of the etching stoplayer 329 exposed from the trenches 344) of the trenches 344, inaccordance with some embodiments. The storage layer 348 is also formedalong the upper surface of the capping layer 343, in accordance withsome embodiments. After forming the storage layer 348, the remainingportions of the trenches 344 are denoted as trenches 344′, in accordancewith some embodiments.

In some embodiments, the storage layer 348 is an ONO storage layer. Insome embodiments, the ONO storage layer includes an oxide-nitride-oxideconfiguration such as SiO₂-SiN-SiO₂. In some embodiments, the storagelayer 348 is formed using CVD (such as LPCVD, PECVD, HDP-CVD, HARP, andFCVD), ALD, another suitable method, or a combination thereof.

A spacer layer 350 is formed over the semiconductor memory device 300,as shown in FIGS. 3G-1, 3G-2, and 3G-3, in accordance with someembodiments. The spacer layer 350 is conformally formed along thestorage layer 348, in accordance with some embodiments. The spacer layer350 is used to protect the storage layer 348 during a subsequent etchingprocess, in accordance with some embodiments. After forming the spacerlayer 350, the remaining portions of the trenches 344′ are denoted astrenches 344″, in accordance with some embodiments.

In some embodiments, the spacer layer 350 is made of a conductivematerial, such as semiconductor material (such as polysilicon orpoly-germanium), TiN, W, Co, or another suitable conductive material. Insome embodiments, the semiconductor material is doped, e.g., with ann-type or a p-type dopant. In some embodiments, the spacer layer 350 isformed using CVD, PVD ALD, e-beam evaporation, or another suitableprocess. In some embodiments, after the material for the spacer layer350 is formed, an etching-back process is performed to remove thematerial for the spacer layer 350 formed above the upper surface of thecapping layer 343.

The bottoms of the trenches 344″ are opened using an etching process, inaccordance with some embodiments. The trenches 344″ are extended fromthe bottoms of the trenches 344″ into and through the spacer layer 350,the storage layer 348, and the etching stop layer 329, as shown in FIGS.3H-1, 3H-2, and 3H-3, in accordance with some embodiments. The enlargedtrenches 344″ are denoted as trenches 345, in accordance with someembodiments. The trenches 345 that are aligned with the sacrificial vias332 exposes the upper surfaces of the sacrificial vias 332, inaccordance with some embodiments. The trenches 345 that are not alignedwith the sacrificial vias 332 expose the upper surface of the IMD layer326M, or alternatively extend into the IMD layer 326M, in accordancewith some embodiments.

In some embodiments, the extension of the trenches 345 is performedwithout the need for a lithography step. That is, in some embodiments,no masking element is formed above the capping layer 343 during theetching process. In some embodiments, the etching process is anisotropicetching such as dry etching. In some embodiments, the etching processhas a non-selectivity between the materials to be etched. In someembodiments, the spacer layer 350 protects the storage layer 348 fromdamage during the etching process. After the removal process, thestorage layer 348 over the upper surface of the capping layer 343 may beremoved.

The sacrificial vias 332 are removed to form via holes 352 in the IMDlayer 326M, as shown in FIGS. 3I-1, 3I-2 and, 3I-3, in accordance withsome embodiments. The via holes 352 expose the upper surfaces of theaccess lines 328M of the top metal layer 328, in accordance with someembodiments. In some embodiments, the removal process includes anetching process, such as dry etching and/or wet etching. Because thematerial for the sacrificial vias 332 has a higher etching rate than thematerial for the IMD layer 326M in the etching process, the IMD layer326M is substantially not etched, in accordance with some embodiments.In some embodiments where the sacrificial vias 332 are made of SiN andthe IMD layer 326M is made of SiO₂-based or low-K material, the etchantof the etching process is hot phosphoric acid (H₃PO₄.). Furthermore, insome embodiments where the insulating layer 342 and the sacrificiallayer 332 are made of nitride (e.g., SiN), the capping layer 343 (e.g.,oxide) protects the insulating layer 342 and underlying material layers(e.g., the source/drain features and channel features) during theetching process.

In some embodiments, the removal process is performed without the needfor a lithography step. That is, in some embodiments, the etchingprocess is a mask-free etching process. As such, the etching process offorming the via holes 352 is a self-aligned etching process, inaccordance with some embodiments. Because the sacrificial vias 332 areformed prior to the memory cells, the via holes 352 may be formedwithout a high aspect ratio etching and the accuracy of the overlay thevia holes 352 to the access line 328M may be relatively high. The highaspect ratio etching may reduce the overlay window of via to access lineand increase the risk of the damage of the storage layer.

The conductive material 354 is formed over the semiconductor memorydevice 300, as shown in FIGS. 3J-1, 3J-2, and 3J-3, in accordance withsome embodiments. The via holes 352 and the trenches 345 arecontinuously filled with the conductive material 354, in accordance withsome embodiments. The conductive material 354 is also formed over theupper surface of the capping layer 343, in accordance with someembodiments. The conductive material 354 filled into the trenches 345forms gate structure 358, and the conductive material 354 filled intothe via holes 352 forms conductive vias 330M, in accordance with someembodiments. Because the gate structures 358 and conductive via 330M area continuous structure formed of the conductive material 345, nointerface is between the gate structure 358 and conductive vias 330M, inaccordance with some embodiments. Furthermore, because the conductivevias 330M are formed by a self-aligned etching process, the overlaywindow of via to access line may be improved and the risk of the damageof the storage layer may be prevented.

The gate structures 358 are used to form gate lines which providecontrol gate for memory cells (discussed below). In some embodiments,the source/drain features 335, 339, the channel feature 346, the storagelayer 348 and the gate line (formed by the gate structure 358) incombination function as a memory transistor T1, e.g., a SONOS-type flashmemory transistor. In some embodiments, the source/drain features 337,341, the channel feature 346, the storage layer 348 and the gate line(formed by the gate structure 358) in combination function as a memorytransistor T2, e.g., a SONOS-type flash memory transistor. In someembodiments, a plurality of pairs of memory transistors T1 and T2vertically stacked share the same gate line.

In some embodiments, the conductive material 354 is semiconductormaterial (such as polysilicon or poly-germanium), TiN, W, Co, or anothersuitable conductive material. In some embodiments, the semiconductormaterial is doped, e.g., with an n-type or a p-type dopant. In someembodiments, the conductive material 354 is the same as the material forthe spacer layer 350. In some embodiments, the conductive material 354is formed using CVD, PVD ALD, e-beam evaporation, or another suitableprocess, or another suitable deposition technique.

The conductive material 354 formed over the upper surface of the cappinglayer 343 is removed, as shown in FIG. 3K, in accordance with someembodiments. In some embodiments, the removal process includes anetching-back process and/or planarization process (e.g., CMP). After theremoval process, the upper surface of the capping layer 343 is exposed,in accordance with some embodiments.

A mask element 360 is formed over the upper surface of the capping layer343 and the upper surface of the gate structures 358, as shown in FIGS.3L-1 and 3L-2, in accordance with some embodiments. In some embodiments,the mask element 360 is a tri-layer mask structure which includes abottom layer 362, a middle layer 364, a top layer 366. In someembodiments, the top layer 366 is a patterned photoresist layer. In someembodiments, the middle layer 364 is made of silicon containing spin-oncoated material. In some embodiments, the bottom layer 362 is made of anorganic spin-on coated material.

The top layer 366 has a plurality of openings 368, in accordance withsome embodiments. The openings 368 are aligned with the gate structure358, as shown in FIGS. 3L-1 and 3L-2, in accordance with someembodiments. The pattern of the openings 368 will be transferred to thegate structure 358 to cut the gate structure 358, in accordance withsome embodiments. The openings 368 are arranged in the X direction andthe Y direction, in accordance with some embodiments. The openings 368over adjacent two gate structures 358 are alternately arranged in the Xdirection in a staggered manner, as shown in FIG. 3L-2, in accordancewith some embodiments.

Furthermore, the openings 368 are staggered with the conductive vias330M, as shown in FIG. 3L-2, in accordance with some embodiments. Theopenings 368 have an X-pitch P_(O-X) (in the X direction) that issubstantially equal to BL pitch (e.g., P_(a2-X)) and a Y-pitch P_(O-Y)(in the Y direction) that is substantially equal to WL pitch (e.g.,P_(a1-Y)), in accordance with some embodiments.

The gate structures 358 are patterned using the mask element 360, inaccordance with some embodiments. The pattern of the openings 368 istransferred to the gate structures 358 using an etching process, inaccordance with some embodiments. The etching process may be a dry etchprocess (e.g., plasma enhanced etch) or a wet etching process. Portionsof the gate structures 358 aligned below the openings 368 are removed toform openings 370 in the gate structures 358, as shown in FIGS. 3M-1,3M-2, and 3M-3, in accordance with some embodiments. After the etchingprocess, the mask element 360 is removed, in accordance with someembodiments.

Each of the gate structures 358 is cut into a plurality of gate lines372 by the openings 370, in accordance with some embodiments. In someembodiments, each of the memory cells C includes a pair of memorytransistors T1 and T2 which are interposed by a single gate line 372.Each gate line 372 extends vertically along within a single memory cellcolumn. In some embodiments, the plurality of pairs of the memorytransistors T1 and T2 (e.g., memory cells C) are vertically arrayed intoa single memory cell columns, and a plurality of memory cell columns arehorizontally arrayed into one or more 3D memory cell arrays. As such,the gate lines 372 define the horizontal configuration of the memorytransistors T1 and T2, in accordance with some embodiments. Theconductive vias 330M are connected to select gate lines 372, inaccordance with some embodiments. For example, the conductive via 330Mmay be connected to every other memory cell and in particular, everyother gate line 372 in the X direction.

Furthermore, portions of the spacer layers 350 underlying the openings368 are also removed. If the portions of spacer layers 350 underlyingthe openings 368 are not removed completely, the adjacent gate lines372, formed by the same gate structure 358, may be electricallyconnected, which may degrade the performance of the semiconductor memorydevice 300.

An insulating material 374 is formed over the semiconductor memorydevice 300, as shown in FIGS. 3N-1, 3N-2, and 3N-3, in accordance withsome embodiments. The insulating material 374 is filled into theopenings 370 to form isolation features 376, in accordance with someembodiments. The isolations feature 376 is used to electrically isolatethe gate lines 372 from one another, in accordance with someembodiments. The insulating material 374 is also formed over the uppersurface of the capping layer 343 and upper surfaces of the gate lines372, in accordance with some embodiments.

In some embodiments, the insulating material 374 is a dielectricmaterial, such as an oxide (e.g., SiO₂), a nitride (SiN), or anothersuitable dielectric material. In some embodiments, the insulatingmaterial 374 is formed using CVD (such as LPCVD, PECVD, HDP-CVD, HARP,and FCVD), ALD, another suitable method, or a combination thereof.

The insulating material 374, formed over the upper surface of thecapping layer 343, may be used as an IMD layer, and is hereinafterreferred to as an IMD layer 375, as shown in FIGS. 3Q-1, 3Q-2, and 3Q-3.A mask element 378 is formed over the upper surface of the IMD layer375, as shown in FIGS. 3O-1, 3O-2, and 3O-3, in accordance with someembodiments. In some embodiments, the mask element 378 is a tri-layermask structure which includes a bottom layer 380, a middle layer 382,and a top layer 384. In some embodiments, the top layer 384 is apatterned photoresist. In some embodiments, the middle layer 382 is madeof silicon containing spin-on coated material. In some embodiments, thebottom layer 380 is made of an organic spin-on coated material.

In some embodiments, the top layer 384 has a plurality of openings 386,as shown in FIGS. 3O-1, 3O-2 and 3L-3. The openings 386 are aligned withthe gate line 372, in accordance with some embodiments. The pattern ofthe openings 382 will be transferred to the IMD layer 375 to define thepattern of conductive vias formed above the memory cell array, inaccordance with some embodiments. The conductive vias provideinterconnection to the memory cell columns and, in particular, aninterconnection to select ones of the gate lines 372 which are notconnected to the conductive vias 330M, in accordance with someembodiments. In some embodiments, the openings 382 have a Y-pitchP_(V2-Y) (in the Y direction) and an X-pitch P_(V2-X) (in the Xdirection).

The IMD layer 375 is patterned using the mask element 378, in accordancewith some embodiments. The pattern of the openings 378 is transferred tothe IMD layer 375 using an etching process, in accordance with someembodiments. The etching process may be a dry etch process (e.g., plasmaenhanced etch) or a wet etching process. Portions of the IMD layer 375aligned below the openings 386 are removed to form via holes 388 in theIMD layer 375, as shown in FIGS. 3P-1, 3P-2, and 3P-3, in accordancewith some embodiments. In some embodiments, the openings 388 expose anupper surface of the gate lines 372. After the etching process, the maskelement 378 is removed, in accordance with some embodiments.

The conductive vias 390 are formed in the via holes 388 within the IMDlayer 375, as shown in FIGS. 3Q-1, 3Q-2, and 3Q-3, in accordance withsome embodiments. The conductive vias 390 are formed interfacing selectgate lines 372, in accordance with some embodiments. For example, theconductive via 390 may interface every other memory cell and inparticular, every other gate line 372 in the X direction.

In some embodiments, the conductive vias 390 have a Y-pitch P_(V2-Y) (inthe Y direction) between adjacent conductive vias 390 that issubstantially equal to pitch P_(a1-Y) of the access lines 328M. In someembodiments, the conductive vias 390 have an X-pitch P_(V2-X) (in the Xdirection) between adjacent conductive vias 390. In some embodiments,the X-pitch P_(V2-X) is twice the BL pitch P_(a2-X). This relaxedX-pitch P_(V2-X) is due to the configuration of half of the memory cellsbeing configured to connect to the access line 328M under the memorycell array of through the and half of the memory cells being configuredto connect to the metal access line above the memory cell array.

In some embodiments, the conductive vias 390 are made of a conductivematerial, such as tungsten (W), molybdenum (Mo), titanium (Ti), cobalt(Co), tantalum (Ta), nickel (Ni), polysilicon, aluminum (Al), copper(Cu), silicides, nitrides, and/or another suitable material. In someembodiments, the conductive vias 390 are formed using a depositionprocess (e.g., CVD, PVD, e-beam evaporation, electroplating, etc.)followed by a planarization process (e.g., CMP).

An IMD layer 391 is formed over the upper surface of the IMD layer 375and the upper surfaces of the conductive vias 390, as shown in FIGS.3R-1, 3R-2, and 3R-3, in accordance with some embodiments. The metallayer 392 is formed in the IMD layer 391, in accordance with someembodiments. The metal layer 392 includes access lines 392N (or referredto top access lines 392N), in accordance with some embodiments. Theaccess lines 392N are formed over and contact the conductive vias 390,in accordance with some embodiments. The access lines 392N extend in theX direction, in accordance with some embodiments. The access lines 392Nare arranged in the Y direction and spaced from one another, inaccordance with some embodiments. The access lines 392N are conductivelines operable to access and/or control other half of memory cells ofthe memory cell array, in accordance with some embodiments.

In some embodiments, the access lines 392N are word lines (WL) and havea pitch P_(a1-Y) in the Y direction between adjacent word lines. In someembodiments, the access lines 392N and 328M provide the samefunctionality (e.g., both provide word lines). In some embodiments, halfof the gate lines 372 are connected to the top access line 392N andother half of the gate lines 372 are connected to the bottom accesslines 328M. In some embodiments, every other gate line 378 (in thex-direction) is connected to the top access line 392N, and the remaininggate lines 378 (in the x-direction) are connected to the bottom accesslines 328M, as shown in FIGS. 3R-1 and 3R-2. In some embodiments, theaccess lines 392N are provided to a first grouping of memory cells, andthe access lines 328M are provided to a second grouping of memory cells,as described above with respect with FIGS. 2A and 2B. In someembodiments, the access lines 392N are bit lines (BL). It should benoted that whether the access lines 392N are word lines or bit lines isdependent on the desired configuration of the memory cells.

In some embodiments, the material and the formation of the IMD layer 391is the same or similar to that of the IMD layers 326. In someembodiments, the metal layer 392N are made of a conductive material,such as tungsten (W), molybdenum (Mo), titanium (Ti), cobalt (Co),tantalum (Ta), nickel (Ni), polysilicon, aluminum (Al), copper (Cu),silicides, nitrides, and/or another suitable material. In someembodiments, the formation of the metal layer 392 is the same as orsimilar to that of the metal layer 328.

Additional IMD layers and additional metal layers and vias may be formedover the IMD layer 391. In addition, the top access lines 392N routed onthe metal layer 392 may be interconnected to the peripheral circuitcomponent (e.g., logic devices 312A and 312B) through an interconnectstructure (e.g., the interconnect structure 320) in another region (notshown) of the substrate 302.

As described above, the semiconductor memory device 300 includes thebottom access lines 328M that are disposed under a memory cell array andprovided to select ones of the memory cells C, in particular, half ofthe gate lines 372, through the conductive vias 330M, and thus the pitchrequired for the access lines 328M is relaxed, which allows for improvedscalability of the array, in accordance with some embodiments.

FIG. 4-1 is a cross-sectional view of a semiconductor memory device 300,in accordance with some embodiments of the disclosure. FIG. 4-2 is anenlarged view of area A shown in FIG. 4-1, in accordance with someembodiments of the disclosure.

In some embodiments, the conductive vias 330M (formed from thesacrificial vias 332) have a width W1 and the gate lines 372 have awidth W2 that is less than the width W1 thereby improving the gate lineto via overlay window. Furthermore. air voids 402 are formed in theconductive material of the conductive vias 330M during the filling ofthe conductive material 354 into the via holes 352, in accordance withsome embodiments. The air voids 402 has a dimension (e.g., the maximumwidth) W2 that is about 0.5 to about 1.5 of the difference between widthW1 and width W2 (W1 minus W2). In some embodiments, the dimension W2 ofthe air voids 402 is small enough, and thus the resistance of theconductive vias 330M does not substantially change.

As described above, the method for forming a semiconductor memory deviceincludes forming a sacrificial via 332 in an IMD layer 326M over asubstrate 302, forming a stack of layers 334 for a memory cell arrayover the IMD layer 326M, forming a trench 345 through the stack oflayers 334 and corresponding to the sacrificial via 332, removing thesacrificial via 332 to form a via hole 352 in the IMD layer 326M, andfilling the trench 345 and the via hole 352 with a conductive material354 to form a gate line 372 and a conductive via 330M. The sacrificialvia 332 is formed prior to the memory cells, and the sacrificial via 332is removed without the need for a lithography step. As a result, theoverlay window of via to access line can be improved, high aspect ratioetching can be avoided, and the risk of the damage of the storage layerof the memory transistors may be prevented. Therefore, the productionyield of the semiconductor memory device can be increased.

Embodiments of a method for forming a semiconductor memory device may beprovided. The method may include forming a sacrificial via in adielectric layer over a substrate, forming a stack of layers for amemory cell array over the dielectric layer, forming a trench throughthe stack of layers and corresponding to the sacrificial via, removingthe sacrificial via to form a via hole in the dielectric layer, andfilling the trench and the via hole with a conductive material. As aresult, the overlay window of via to access line may be improved, highaspect ratio etching may be avoided, and the risk of the damage of thestorage layer of the semiconductor memory device may be prevented.Therefore, the production yield of the semiconductor memory device maybe increased.

In some embodiments, a method for forming a semiconductor memory deviceis provided. The method includes forming a sacrificial via in adielectric layer over a substrate, forming a first active layer over thedielectric layer, forming an insulating layer over the first activelayer, and forming a second active layer over the insulating layer. Themethod also includes forming a trench through the second active layer,the insulating layer and the first active layer and corresponding to thesacrificial via, removing the sacrificial via to form a via hole in thedielectric layer, and filling the trench and the via hole with aconductive material.

In some embodiments, a method for forming a semiconductor memory deviceis provided. The method includes forming an interconnect structure overa substrate. The interconnect structure includes an access line in adielectric layer. The method also includes forming a sacrificial via inthe dielectric layer and on the access line, and forming a memory cellarray over the interconnect structure. Forming the memory cell arrayincludes forming a stack of layers over the dielectric layer. The stackof layers includes active layers interposed by an insulating layer.Forming the memory cell array also includes cutting the stack of layersto form first source/drain features and second source/drain featuresspaced apart from the first source/drain features by a trench, etchingthe sacrificial via through the trench to form a via hole exposing theaccess line, and depositing a conductive material in the trench and thevia hole.

In some embodiments, a semiconductor memory device is provided. Thesemiconductor memory device includes a logic device on a substrate, andan interconnect structure over the logic device. The interconnectstructure includes an access line and a conductive via on the firstaccess line. The semiconductor memory device also includes a memory cellover the interconnect structure, and a gate line vertically extendingalong the memory cell and above the conductive via. The gate lineconnects the memory cell to the first access line. The conductive viaand the gate line are formed of a continuous conductive material.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A method for forming a semiconductor memorydevice, comprising: forming a sacrificial via in a dielectric layer overa substrate; forming a first active layer over the dielectric layer;forming an insulating layer over the first active layer; forming asecond active layer over the insulating layer; forming a trench throughthe second active layer, the insulating layer and the first active layerand corresponding to the sacrificial via; removing the sacrificial viato form a via hole in the dielectric layer; and filling the trench andthe via hole with a conductive material.
 2. The method for forming thesemiconductor memory device as claimed in claim 1, wherein removing thesacrificial via comprises performing a mask-free etching process.
 3. Themethod for forming the semiconductor memory device as claimed in claim1, further comprising, before removing the sacrificial via: forming astorage layer along a sidewall and a bottom surface of the trench; andforming a spacer layer on the storage layer.
 4. The method for formingthe semiconductor memory device as claimed in claim 3, furthercomprising: partially removing the spacer layer and the storage layer toexpose the sacrificial via.
 5. The method for forming the semiconductormemory device as claimed in claim 1, furthering comprising: beforeforming the first active layer over the dielectric layer, forming anetching stop layer covering the sacrificial via, wherein the trenchexposes an upper surface of the etching stop layer.
 6. The method forforming the semiconductor memory device as claimed in claim 5,furthering comprising: partially removing the etching stop layer toexpose the sacrificial via.
 7. The method for forming the semiconductormemory device as claimed in claim 1, wherein the first active layer andthe second active layer are cut by the trench to form source/drainfeatures at a side of the trench and interposed by the insulating layer.8. The method for forming the semiconductor memory device as claimed inclaim 7, further comprising: forming a channel feature on a sidewall ofthe insulating layer exposed from the trench.
 9. The method for formingthe semiconductor memory device as claimed in claim 1, wherein the viahole is partially filled by the conductive material.
 10. A method forforming a semiconductor memory device, comprising: forming aninterconnect structure over a substrate, the interconnect structureincluding an access line in a dielectric layer; forming a sacrificialvia in the dielectric layer and on the access line; and forming a memorycell array over the interconnect structure, comprising: forming a stackof layers over the dielectric layer, the stack of layers includingactive layers interposed by an insulating layer; cutting the stack oflayers to form first source/drain features and second source/drainfeatures spaced apart from the first source/drain features by a trench;etching the sacrificial via through the trench to form a via holeexposing the access line; and depositing a conductive material in thetrench and the via hole.
 11. The method for forming the semiconductormemory device as claimed in claim 10, wherein the sacrificial via ismade of a dielectric material, and etching the sacrificial via comprisesperforming a wet etching process.
 12. The method for forming thesemiconductor memory device as claimed in claim 10, wherein theconductive material is polysilicon, and the conductive material iscontinuously deposited in the via hole and the trench.
 13. The methodfor forming the semiconductor memory device as claimed in claim 10,wherein the sacrificial via has a first width and the trench has asecond width that is less than the first width.
 14. The method forforming the semiconductor memory device as claimed in claim 10, whereinan air void is formed in the conductive material of the first conductivevia.
 15. The method for forming the semiconductor memory device asclaimed in claim 10, wherein forming the memory cell array over theinterconnect structure further comprises: forming a capping layer overthe stack of layers, wherein the capping layer protects the stack oflayers during etching the sacrificial via through the trench.
 16. Asemiconductor memory device, comprising: a logic device on a substrate;an interconnect structure over the logic device, comprising an accessline and a conductive via on the access line; a memory cell over theinterconnect structure; and a gate line vertically extending along thememory cell and above the conductive via, wherein the gate line connectsthe memory cell to the access line, wherein the conductive via and thegate line are formed of a continuous conductive material.
 17. Thesemiconductor memory device as claimed in claim 16, wherein theconductive via is wider than the gate line.
 18. The semiconductor memorydevice as claimed in claim 16, wherein the memory cell comprises flashmemory transistors which are interposed by the gate line.
 19. Thesemiconductor memory device as claimed in claim 16, wherein theconductive via has an air void in the conductive material.
 20. Thesemiconductor memory device as claimed in claim 19, wherein theconductive via has a first width, the gate line has a second width, andthe air void has a third width, and the third width is 0.5 to 1.5 of thedifference between the first width and the second width.